N.H. firm makes a 'battery' out of air, to power hundreds of homes at once

Sep 4 - McClatchy-Tribune Regional News - David Brooks The Telegraph, Nashua, N.H.

 

Forget mousetraps: If you can build a better battery, the energy-hungry world will really beat a path to your door.

A New Hampshire startup spun out of Dartmouth College's business school thinks it can do that, using compressed air.

It's called SustainX (despite how it seems, putting the letter "X" in a startup's name is not mandatory; it's just a quick way to turn an English word into a trademark-able name).

The firm launched in 2007, and in late August, it unveiled a unit that looks like an enormous engine but acts like a battery. It uses electricity to drive a crankshaft system that compresses air to 3,000 pounds per square inch, a hundred times the pressure in the average car tire.

It holds the air until the electricity is needed again, at which point it expands the air to drive a 1.5-megawatt generator -- enough to power hundreds of homes for several hours.

Large-scale electricity storage is needed largely because solar and wind power is intermittent, as compared to always-on fossil fuel or nuclear power plants. Storing solar power for use at night or wind power for use in calm periods will be needed if we're going to transition away from an electric grid that generates greenhouse gases.

Batteries, which tap into the flow of electrons between various liquids or solids, are expensive and difficult to scale up to sizes needed by power companies, so other technologies are being examined. They include the flywheel system pioneered by former Tyngsborough, Mass., firm Beacon Power (which has declared bankruptcy but isn't dead); pumped water storage using dams; and even distributed systems that can tweak the temperature of people's hot water tanks to turn them into a type of battery.

Compressed-air energy storage has been around for a while, but SustainX thinks its technology, especially the way it takes advantage of heat loss associated with compressing and expanding gasses, makes the process economical at large scale.

"This is a variation on a technology that was first developed in the 1970s," said Gene Hunt, director of communications for SustainX. "Air is compressed by a compression engine and stuffed into underground caverns when, say, the electricity is cheap, then expanded to run a generator when electricity is more expensive, or when it's needed."

Two commercial-scale systems use caverns, one in Germany and one in Tennessee, but it can't happen here because our geology left New England with few caves of any size, let alone caverns. There is also a question of efficiency.

"When you compress air, you generate heat. Traditional systems vent it off ... then when the air is expanded, it's done via gas turbine," Hunt said.

As a result, he said, compressed air energy storage in caverns "has been characterized as a more efficient way to use natural gas."

A key part of SustainX's approach, Hunt said, is to capture the heat released during compression in a water-foam mixture, then use it to reheat the air when the time comes to use it. They claim their isothermal (unchanged temperature) approach is 95 percent thermally efficient.

The drawback is size: Although a 1.5-megawatt unit seems huge, it's tiny by utility standards. The smallest wind farm in New Hampshire, Lempster Mountain, generates 25 megawatts at full power. If you don't have a free underground cavern to use for storage, you're never going to be able to build a compressed-air system big enough to replace a power plant.

Since being launched by three people in the doctoral program at the Thayer School at Dartmouth, SustainX got funding from various sources, including the National Science Foundation, for demonstrations in the laboratory. They then built a 40-kilowatt prototype (a typical home setup of solar panels generates 5 kilowatts).

The firm raised about $25 million in venture capital funding and got a $5.4 million stimulus grant from the Department of Energy, which is eager to see more energy-storage options developed. That led to development of the megawatt-scale system.

It came to the Seacoast area for a variety of reasons, including tapping into the technical knowledge of employees, Hunt said.

Although wind farms and solar farms are the most high-profile users of energy storage, Hunt said another market is to shift energy production to eliminate or reduce the need for new transmission lines -- heading off Northern Pass-type debates. This may require some reworking of federal energy regulations, which deal with electricity production and electricity transmission, but not electricity storage.

The most immediate markets, Hunt said, are overseas, in fast-growing economies where the need for power has outpaced the grid. Some well-placed storage can ward off brownouts.

"The markets for bulk energy storage elsewhere, at the moment, are more attractive than in the U.S.," he said.

David Brooks can be reached at 594-6531 or dbrooks@nashua telegraph.com. Also, follow Brooks on Twitter (@GraniteGeek).

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